riscv: configs: AE350 will use CONFIG_OF_PRIOR_STAGE when boots from ram

When AE350 boots from ram, use CONFIG_OF_PRIOR_STAGE instead
of CONFIG_OF_BOARD.

Signed-off-by: Rick Chen <rick@andestech.com>
Cc: Greentime Hu <greentime@andestech.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>